1. Field of the Invention
The present invention relates to a method for manufacturing a magnetic memory device and a magnetic memory device. More particularly, the present invention is concerned with a method for manufacturing a nonvolatile magnetic memory device and a magnetic memory device which stores information by utilizing a change of the resistance value caused by changing of the spin direction of a ferromagnetic material to be parallel or non-parallel.
2. Description of Related Art
As information communication devices, especially small devices for personal use, such as mobile terminals, are rapidly spreading, there are demands for devices constituting such electronic devices, such as a memory element and a logic element, which have higher performance, for example, higher degree of integration, higher speed, and lower power consumption. Particularly, a nonvolatile memory is considered indispensable in the ubiquitous era.
For example, even when consumption of power supply or trouble thereof occurs, or disconnection of a server and a network occurs due to certain interference, a nonvolatile memory can protect important personal information. In addition, the increase of the density and capacity of the nonvolatile memory is more important as a technique of substitute for a hard disk or an optical disk which fundamentally cannot be miniaturized due to the presence of moving parts.
Recently available portable devices are designed so that a non-operating circuit block is maintained in a standby state to reduce the power consumption to a possible lowest level, and the waste of power consumption and memory can be avoided if a nonvolatile memory capable of serving as both a high speed network memory and a large storage capacity memory can be realized. Further, when the high-speed large-capacity nonvolatile memory can be realized, a function such that a device works the instance it is turned on, i.e., so-called “instant-on” function may be possible.
Examples of nonvolatile memories include a flash memory using a semiconductor and a ferroelectric random access memory (FERAM) using a ferroelectric material. However, the flash memory has a disadvantage in that the write speed is as low as the order of microsecond. In addition, the flash memory has also disadvantages in that the structure is complicated and hence the increase of the degree of integration is difficult, and that the access time is as slow as about 100 ns. On the other hand, with respect to the FRAM, the endurance is 1012 to 1014, and a problem has been pointed out such that the endurance is not sufficient to completely replace the existing memory by a static random access memory or a dynamic random access memory. Further, another problem that micro-fabrication of a ferroelectric capacitor is difficult is also pointed out.
As a nonvolatile memory free of the above problems, a magnetic memory called magnetic random access memory (MRAM) or magnetoresistance (MR) memory is in the limelight and, recently, tunnel magnetoresistance (hereinafter, frequently referred to simply as “TMRI”) effect element materials are being improved in properties and hence the magnetic memory has attracted greater attention (see, for example, Non-patent document 1). In addition, the MRAM has a memory element formed in a wiring portion, and therefore is advantageous in that the degree of freedom for mounting of the memory portion is high, integration is easy, mounting in combination with a logic circuit is easy, the MRAM has compatibility with a complementary metal oxide semiconductor (CMOS) process, and the like.
The MRAM has a simple structure and is easy to increase the degree of integration, and stores data by utilizing rotation of a magnetic moment and therefore is expected to have higher endurance. In addition, it is expected that the access time of MRAM is very fast, and it has already been reported that the MRAM can be operated at 100 MHz (see, for example, Non-patent document 2). Further, the MRAM has been remarkably improved since a higher power can be currently obtained due to a giant magnetoresistance (GMR) effect.
Differing from a conventional memory function utilizing electrons (electricity), the MRAM is a device using as a memory medium a change of the magnetoresistance caused by changing of the direction of magnetization, which needs to operate the response of changing of the direction of magnetization at a speed equivalent to the speed of the response of the conduction of electrons. The direction of magnetization of the MRAM changes depending on the current which flows a metal wiring. That is, a current flows a wiring to generate a magnetic field in the center of the wiring. An MRAM element {TMR or magnetic tunnel junction (MTJ)} detects the magnetic field generated, so that the magnetic material in the MRAM element is magnetized in the direction linked to the direction of the magnetic field generated in the wiring. The magnetic material magnetized causes a magnetoresistance, and the magnetoresistance is read as a change of voltage or current. It is important that the magnetic field generated is efficiently introduced to the MRAM element, and this efficiency is considered to determine the operation speed and sensitivity-of the MRAM element. Factors for efficiently introducing the magnetic field to the MRAM element include: (I) generation of an intense magnetic field; (II) suppression of leakage of the magnetic field; (III) arrangement of the MRAM element in the intense magnetic field portion; (IV) high sensitivity of the MRAM element, and the like.
With respect to the factor (I), the intensity of a magnetic field depends on the current density, and, as the current density of a wiring increases, the intensity of the magnetic field increases. The increase of the current density promotes electromigration of the wiring, and hence an aluminum wiring is not used but a copper wiring is used to improve the intensity of the magnetic field. With respect to the factor (III), the problem is solved by arranging a wiring and the MRAM element so that they are close to each other. With respect to the factor (IV), the problem is solved by improving the material for and method for forming the MRAM element.
With respect to the factor (II), a detailed explanation is made. Storage in the MRAM is made by rotating the magnetization of the memory layer utilizing a current magnetic field generated by allowing a current to flow a wiring. However, as the wiring becomes thinner due to an increase of the degree of integration, the critical value of a current which can flow the writing line is lowered, so that only a weak magnetic field can be obtained, thus inevitably reducing the coercive force of a region in which data is stored. This means that the reliability of the information memory device is lowered. In addition, unlike a light or an electron beam, a magnetic field cannot be focused and this is considered to be the biggest cause of cross talk when the degree of integration is increased. For preventing this, a keeper structure and the like have been proposed, but they inevitably cause the structure to be complicated. As described above, writing using a current magnetic field has a number of fundamental problems to be solved, and the writing using a current magnetic field may be a great defect of the future MRAM.
With respect to the factor (II), an attempt is made to solve the problem by a method using a cladding structure in which a wiring portion is covered with a soft magnetic material. It is noted that the wiring is not completely covered, and the soft magnetic material is not formed on the surface on the side of the MRAM element since a magnetic field must be supplied to the MRAM element from the plane of the wiring facing the MRAM element. Specifically, a word line is generally provided under the MRAM element, and hence no soft magnetic material is formed on the word line. A bit line is provided on the MRAM element, and hence no soft magnetic material is formed under the bit line (see, for example, Non-patent document 1).
With respect to the formation of a cladding structure, several methods have been proposed, and the most general method is described below. This is a method for forming a cladding structure for word line.
The following procedure is not shown in the figure. (a): On the inner wall of a wiring trench formed in an insulating film, in which a word line is formed, a barrier metal layer, a soft magnetic material layer, and a copper seed layer are deposited by, for example, sputtering. (b): The wiring trench is filled with a wiring material by a plating process, a chemical vapor deposition process or the like. (c): The excess wiring material formed on the insulating film is removed by chemical mechanical polishing so that the wiring material remains only in the wiring trench and the surface of the insulating film is planarized, thereby forming a word line comprised of the wiring material remaining in the wiring trench.
On the other hand, the method for forming a cladding structure for bit line is complicated. One example of the method is described with reference to the diagrammatic cross-sectional views of FIGS. 9A to 9J. As shown in FIG. 9A, an element for selection (not shown), a sense line (not shown), and the like are formed on a substrate (not shown), and a first insulating film 41 is formed so as to cover them. On the first insulating film 41 is formed a second insulating film 42 in which a word line and the like are formed, and in the second insulating film 42 are formed a word line 11 having a trench wiring structure, an electrode (not shown) connected to the element for selection (not shown), and the like. On the second insulating film 42, a memory element 13 is formed above the word line 11 through a third insulating film 43 for covering the word line 11 and the electrode. The memory element 13 is comprised of, for example, a TMR element. Under the memory element 13, a by-pass line 17 comprised of an antiferromagnetic layer, a conductive layer or the like is formed and connected to the electrode. On the third insulating film 43, a forth insulating film 44 is formed so as to cover the memory element 13, and then the forth insulating film 44 is subjected to planarization so that the upper surface of the memory element 13 is exposed. Then, on the forth insulating film 44, a fifth insulating film 45 for covering the memory element 13, in which a bit line is formed, is deposited, and then a wiring trench 46 in which a bit line is formed is formed so that the upper surface of the memory element 13 is exposed to the bottom of the wiring trench and a contact hole (not shown) to the word line 11 is formed.
Then, as shown in FIG. 9B, a barrier metal layer 121 and a soft magnetic material layer 122 are successively deposited on the wiring trench 46 and the contact hole. The barrier metal layer 121 is deposited by a sputtering process, a chemical vapor deposition (hereinafter, frequently referred to simply as “CVD”) process, an atomic layer deposition (ALD) process or the like, and the deposition method is selected depending on the form and size of the wiring trench in which the barrier metal layer 121 is formed. In the barrier metal layer 121, tantalum (Ta), tantalum nitride (TaN), titanium (Ti), titanium nitride (TiN), tungsten nitride (WN), zirconium nitride (ZnN), or the like may be used, and the thickness of the layer is advantageously in the range of from 5 to 50 nm. The soft magnetic material layer 122 is deposited by a sputtering process or the like. The thickness of the soft magnetic material layer 122 is required to be adjusted by changing the amount of the below-described etchback. Specifically, the etchback must meet both the requirement that the soft magnetic material layer 122 on the bottom of the wiring trench 46 be completely removed and the requirement that the soft magnetic material layer 122 remaining on the sidewall of the wiring trench 46 have a thickness sufficient to exhibit a magnetic flux focusing effect. This is affected by not only the thickness of the soft magnetic material layer 122 but also the coverage of the deposition method. For example, in a sputtering process, the side coverage is several tens % at most and the bottom coverage is several tens %. The side coverage is poor, and therefore it is required to deposit a film having a large thickness to a certain extent.
Then, as shown in FIG. 9C, the soft magnetic material layer deposited on the bottom of the wiring trench is removed (generally employing etchback by dry etching). In this step, it is important that the soft magnetic material layer 122 on the bottom of the wiring trench 46 is completely removed and that the soft magnetic material layer 122 remains on the sidewall of the wiring trench so that the thickness is sufficient to exhibit a magnetic flux focusing effect. This step generally employs anisotropic etching, and etching having high degree of anisotropy is desired since the soft magnetic material layer 122 must remain on the sidewall of the wiring trench 46, and a plasma etching technique which generates high-density etching species, such as inductively coupled plasma (ICP) or electron cyclotron resonance (ECR), is used.
Then, as shown in FIG. 9D, a barrier metal layer 123 is formed on the inner walls of the wiring trench 46 and the contact hole, and then a copper seed layer 124 is deposited thereon. The copper seed layer 124 is deposited using, for example, a sputtering process or a CVD process. The deposition method and the thickness of the layer are appropriately selected depending on the form and size of the wiring trench 46 in which a bit line is formed and the contact hole.
Further, as shown in FIG. 9E, the wiring trench 46 and the contact hole are filled with a wiring material 125 by, for example, an electro-chemical deposition (ECD) process or a CVD process.
Then, as shown in FIG. 9F, the excess wiring material 125 (including the copper seed layer 124) and barrier metal layers 123, 121 {see FIGS. 9B and 9D} on the fifth insulating film 45 are removed by chemical mechanical polishing so that the wiring material 125 remains in the wiring trench 46 and the contact hole, thereby forming a bit line 12 so that the wiring material 125, the barrier metal layers 123, 121, and the soft magnetic material layer 122 remain in the wiring trench 46 and the contact hole and planarizing the surface.
Then, as shown in FIG. 9G, on the fifth insulating film 45, a soft magnetic material layer 126 for covering the bit line 12 is deposited. The deposition of the soft magnetic material layer 126 is similar to the method described above. The material for the soft magnetic material layer 126 may contain atoms which diffuse into the device, and, in such a case, it is preferred to form a barrier metal layer under the soft magnetic material layer 126.
Then, as shown in FIG. 9H, a resist mask 51 for covering the bit line 12 and forming a magnetic material layer pattern is formed on the bit line 12 by resist application and a lithography technique. The resist mask 51 is required to completely cover the bit line 12. The reason for this is that magnetic flux leakage is caused from the portion in which the soft magnetic material layer 126 is not formed and the occurrence of magnetic flux leakage is concentrated in the magnetic flux leakage portion. The resist mask at an inappropriate position due to misalignment causes magnetic flux leakage, and therefore there is need to provide a margin in the reticle mask so that the soft magnetic material layer 126 covers the bit line 12 even when misalignment occurs.
Then, as shown in FIG. 9I, the soft magnetic material layer 126 is processed by dry etching using the resist mask 51, and then the resist film used as the resist mask 51 is peeled off, followed by cleaning, thereby forming the soft magnetic material layers 122, 126 covering the side and upper portions of the bit line 12 as shown in FIG. 9J.
In the etching for the soft magnetic material layer 126 using the resist mask 51, the resist mask 51 may have a problem of durability. In such a case, a hard mask comprised of a silicon oxide film or a silicon nitride film is used. For example, a process is employed in which the soft magnetic material layer 126 is deposited, and then a silicon oxide film or a silicon nitride film is deposited as a hard mask, and the hard mask is processed using the resist mask and the resist is peeled off, followed by etching for the soft magnetic material.
As described above, the formation of a cladding structure for word line can be practiced by a simple process and has no problem.
[Patent Document 1]
Unexamined Japanese Patent Application Laid-Open Specification No. 2002-246566 (FIG. 6 appearing at page 4)
[Non-patent Document 1]
Wang et al., IEEE Trans. Magn. 33 (1997), p. 4,498–4,512
[Non-patent Document 2]
R. Scheuerlein et al., ISSCC Digest of Papers (February 2000), p. 128–129
However, the formation of a cladding structure for bit line has several problems to be solved. The problems are listed below. A process having the number of steps as large as eight is needed. In the removal of the soft magnetic material on the bottom of the wiring trench by an etchback process using dry etching while allowing the soft magnetic material to remain on the sidewall of the wiring trench, there are problems of stability and margin of the process. For forming the soft magnetic material layer on a wiring, the soft magnetic material layer is deposited directly on an interlayer dielectric, and therefore there occurs a problem in that the interlayer dielectric suffers contamination. When employing a soft magnetic material/barrier metal structure for preventing contamination, a burden of dry etching and deterioration of flatness of the bit line are considered. For shielding a magnetic field, the bit line is required to be completely covered with the soft magnetic material. For this reason, a mask having alignment tolerance for the lithography step is needed. In other words, this process is disadvantageous for shrinking. Thus, the formation of a cladding structure for bit line has serious problems to be solved.